fix(pg_money): handle negative values correctly in PgMoney::from_decimal() (#1334)

closes #1321

sqlx-core/src/postgres/types/money.rs

@@ -6,6 +6,7 @@ use crate::{
     types::Type,
 };
 use byteorder::{BigEndian, ByteOrder};
+use std::convert::TryFrom;
 use std::{
     io,
     ops::{Add, AddAssign, Sub, SubAssign},
@@ -20,46 +21,100 @@ use std::{
 ///
 /// Reading `MONEY` value in text format is not supported and will cause an error.
 ///
+/// ### `locale_frac_digits`
+/// This parameter corresponds to the number of digits after the decimal separator.
+///
+/// This value must match what Postgres is expecting for the locale set in the database
+/// or else the decimal value you see on the client side will not match the `money` value
+/// on the server side.
+///
+/// **For _most_ locales, this value is `2`.**
+///
+/// If you're not sure what locale your database is set to or how many decimal digits it specifies,
+/// you can execute `SHOW lc_monetary;` to get the locale name, and then look it up in this list
+/// (you can ignore the `.utf8` prefix):
+/// https://lh.2xlibre.net/values/frac_digits/
+///
+/// If that link is dead and you're on a POSIX-compliant system (Unix, FreeBSD) you can also execute:
+///
+/// ```sh
+/// $ LC_MONETARY=<value returned by `SHOW lc_monetary`> locale -k frac_digits
+/// ```
+///
+/// And the value you want is `N` in `frac_digits=N`. If you have shell access to the database
+/// server you should execute it there as available locales may differ between machines.
+///
+/// Note that if `frac_digits` for the locale is outside the range `[0, 10]`, Postgres assumes
+/// it's a sentinel value and defaults to 2:
+/// https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/cash.c#L114-L123
+///
 /// [`MONEY`]: https://www.postgresql.org/docs/current/datatype-money.html
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
-pub struct PgMoney(pub i64);
+pub struct PgMoney(
+    /// The raw integer value sent over the wire; for locales with `frac_digits=2` (i.e. most
+    /// of them), this will be the value in whole cents.
+    ///
+    /// E.g. for `select '$123.45'::money` with a locale of `en_US` (`frac_digits=2`),
+    /// this will be `12345`.
+    ///
+    /// If the currency of your locale does not have fractional units, e.g. Yen, then this will
+    /// just be the units of the currency.
+    ///
+    /// See the type-level docs for an explanation of `locale_frac_units`.
+    pub i64,
+);
 
 impl PgMoney {
-    /// Convert the money value into a [`BigDecimal`] using the correct
-    /// precision defined in the PostgreSQL settings. The default precision is
-    /// two.
+    /// Convert the money value into a [`BigDecimal`] using `locale_frac_digits`.
+    ///
+    /// See the type-level docs for an explanation of `locale_frac_digits`.
     ///
     /// [`BigDecimal`]: crate::types::BigDecimal
     #[cfg(feature = "bigdecimal")]
-    pub fn to_bigdecimal(self, scale: i64) -> bigdecimal::BigDecimal {
+    pub fn to_bigdecimal(self, locale_frac_digits: i64) -> bigdecimal::BigDecimal {
         let digits = num_bigint::BigInt::from(self.0);
 
-        bigdecimal::BigDecimal::new(digits, scale)
+        bigdecimal::BigDecimal::new(digits, locale_frac_digits)
     }
 
-    /// Convert the money value into a [`Decimal`] using the correct precision
-    /// defined in the PostgreSQL settings. The default precision is two.
+    /// Convert the money value into a [`Decimal`] using `locale_frac_digits`.
+    ///
+    /// See the type-level docs for an explanation of `locale_frac_digits`.
     ///
     /// [`Decimal`]: crate::types::Decimal
     #[cfg(feature = "decimal")]
-    pub fn to_decimal(self, scale: u32) -> rust_decimal::Decimal {
-        rust_decimal::Decimal::new(self.0, scale)
+    pub fn to_decimal(self, locale_frac_digits: u32) -> rust_decimal::Decimal {
+        rust_decimal::Decimal::new(self.0, locale_frac_digits)
     }
 
-    /// Convert a [`Decimal`] value into money using the correct precision
-    /// defined in the PostgreSQL settings. The default precision is two.
+    /// Convert a [`Decimal`] value into money using `locale_frac_digits`.
     ///
-    /// Conversion may involve a loss of precision.
+    /// See the type-level docs for an explanation of `locale_frac_digits`.
+    ///
+    /// Note that `Decimal` has 96 bits of precision, but `PgMoney` only has 63 plus the sign bit.
+    /// If the value is larger than 63 bits it will be truncated.
     ///
     /// [`Decimal`]: crate::types::Decimal
     #[cfg(feature = "decimal")]
-    pub fn from_decimal(decimal: rust_decimal::Decimal, scale: u32) -> Self {
-        let cents = (decimal * rust_decimal::Decimal::new(10i64.pow(scale), 0)).round();
+    pub fn from_decimal(mut decimal: rust_decimal::Decimal, locale_frac_digits: u32) -> Self {
+        // this is all we need to convert to our expected locale's `frac_digits`
+        decimal.rescale(locale_frac_digits);
 
-        let mut buf: [u8; 8] = [0; 8];
-        buf.copy_from_slice(&cents.serialize()[4..12]);
+        /// a mask to bitwise-AND with an `i64` to zero the sign bit
+        const SIGN_MASK: i64 = i64::MAX;
 
-        Self(i64::from_le_bytes(buf))
+        let is_negative = decimal.is_sign_negative();
+        let serialized = decimal.serialize();
+
+        // interpret bytes `4..12` as an i64, ignoring the sign bit
+        // this is where truncation occurs
+        let value = i64::from_le_bytes(
+            *<&[u8; 8]>::try_from(&serialized[4..12])
+                .expect("BUG: slice of serialized should be 8 bytes"),
+        ) & SIGN_MASK; // zero out the sign bit
+
+        // negate if necessary
+        Self(if is_negative { -value } else { value })
     }
 
     /// Convert a [`BigDecimal`](crate::types::BigDecimal) value into money using the correct precision
@@ -67,12 +122,14 @@ impl PgMoney {
     #[cfg(feature = "bigdecimal")]
     pub fn from_bigdecimal(
         decimal: bigdecimal::BigDecimal,
-        scale: u32,
+        locale_frac_digits: u32,
     ) -> Result<Self, BoxDynError> {
         use bigdecimal::ToPrimitive;
 
-        let multiplier =
-            bigdecimal::BigDecimal::new(num_bigint::BigInt::from(10i128.pow(scale)), 0);
+        let multiplier = bigdecimal::BigDecimal::new(
+            num_bigint::BigInt::from(10i128.pow(locale_frac_digits)),
+            0,
+        );
 
         let cents = decimal * multiplier;
 
@@ -277,9 +334,25 @@ mod tests {
     #[test]
     #[cfg(feature = "decimal")]
     fn conversion_from_decimal_works() {
-        let dec = rust_decimal::Decimal::new(12345, 2);
+        assert_eq!(
+            PgMoney(12345),
+            PgMoney::from_decimal(rust_decimal::Decimal::new(12345, 2), 2)
+        );
 
-        assert_eq!(PgMoney(12345), PgMoney::from_decimal(dec, 2));
+        assert_eq!(
+            PgMoney(12345),
+            PgMoney::from_decimal(rust_decimal::Decimal::new(123450, 3), 2)
+        );
+
+        assert_eq!(
+            PgMoney(-12345),
+            PgMoney::from_decimal(rust_decimal::Decimal::new(-123450, 3), 2)
+        );
+
+        assert_eq!(
+            PgMoney(-12300),
+            PgMoney::from_decimal(rust_decimal::Decimal::new(-123, 0), 2)
+        );
     }
 
     #[test]